There are different technical concepts for singling stacks of sheets such as bank-note bundles so that the singled bank notes can be supplied to a testing sensor system which determines the authenticity, qualitative nature, denomination or other characteristic properties of the bank notes.
The present application deals with the concept of friction singlers. In a friction wheel singler, the friction elements of a singler roller for example act on the surface of a bank note of a bank-note stack, the thus contacted bank note being conveyed in a transport direction due to friction by rotation of the singler roller while the other bank notes of the bank-note stack are held back by a retaining device. The retaining device and the singler roller to this end form a singler gap through which the bank note is conveyed. The singler roller has grooves extending in the transport direction of the bank notes to be singled and capable of being slightly engaged by the retaining device. The engagement depth is adjustable.
To make sure the bank note contacted by the singler roller is conveyed and the other bank notes of the bank-note stack are held back, the singler roller must exert on the bank note in the singler gap a singling force or feed force that is greater than the retaining force exerted on the bank note by the retaining device on the opposite side of the singler gap. The retaining device can be realized for example as a retaining roller or retaining pad or runner, which can be mounted either rigidly or so as to rotate against the transport direction.
To adjust the ratio of feed force to retaining force to a desired fixed value, the singler roller can be provided with friction elements whose friction linings have a considerably higher coefficient of friction than the corresponding friction linings of the retaining device, the ratio of friction being for example about 2:1.
It has proved disadvantageous in this procedure that the different friction materials of the singler roller and the retaining device partly have very different operating behavior, for example with respect to resistance to environmental influences, moisture absorption, temperature coefficient, aging and wear resistance. This can lead to different service lives and influences the ratio of friction, which can lead to singling errors including a multiple pick, by which more than one sheet is grasped and conveyed by the singler roller.
For avoiding these problems, a friction wheel singler was developed wherein the same friction material, or friction material with the same coefficient of friction, is used for singler roller and retaining device (DE 100 08 135 A1). To make sure the singler roller force acting on the sheet material to be singled is sufficiently greater than the force exerted by the retaining device despite the use of substantially the same friction material for retention and singling, it is provided that the contact area between the sheet material and the friction elements of the singler roller is substantially greater than the contact area between the sheet material and the friction areas of the retaining device. This is obtained by the retaining roller engaging adjacent circumferential grooves of the singler roller alternately with a material of high coefficient of friction (“friction area”) and with a material of low coefficient of friction (“sliding area”). If the coefficient of friction of the sliding areas of the retaining device is negligibly low, a ratio of coefficients of friction of about 2:1 again results because the friction materials of the retaining device and the singler roller are otherwise the same.
The friction wheel singler according to DE 100 08 135 A1 is particularly suitable for transverse singling of bank notes, by which bank notes are singled with their longitudinal edge leading. When the friction wheel singler is used as a longitudinal singler, by which bank notes are singled with their narrower transverse edge leading, bank notes are occasionally skewed during singling, however, which can lead to jamming in the singler gap or to a jam in the subsequent transport path.